It is often highly desirable to design and control the structure and properties of surfaces and interfaces without affecting a material's bulk properties. Surface properties of interest include wettability, adhesion, tack, friction and wear, hardness, and gloss. The ability to modify and control chemical functionality on surfaces in a precise manner is desirable because a change in surface chemistry inevitably changes the surface free energy and other material properties of interest, such as adhesion and wettability, which are important in coatings and paints. It also allows the possibility of further surface derivatization of external ligands, for example, by selectively attaching molecules of interest to polymer surfaces, which is an important way to build molecular assemblies with confined positions for complex nanoscale and biomolecular devices. The task of modifying surfaces and interfaces to control such properties is daunting because many surface characteristics are related to the nature and physical structure on the molecular level. Ideally, surface modification strategies involve few or no reagents, require only ambient conditions in normal atmosphere, are universal for a variety of substrate materials, and do not necessitate elaborate external processing operations. Most current surface modification strategies do not meet these conditions.
A variety of surface modification techniques are known. Few current techniques for surface modification, however, allow for specific chemical and structural control at molecular dimensions. In addition, many methods under development for surface modification are not readily suited to industrial application and scale up.
One common way to achieve modification of polymer surfaces is by plasma, X-ray, UV, laser, ion beam and e-beam etching, or corona discharge, which involves bombarding the surface with highly excited atomic, molecular, ionic, electronic, or free radical species to form reactive groups on an inert surface. Oxidative treatments, such as corona discharge, oxygen plasma, or UV/ozone are indiscriminate and kinetic in nature, rendering the control of modification depth of surface chemistry difficult. These treatments are often unstable, and involve reorganization at the surface shortly after processing. Chemical treatment methods for surface modification often require harsh and hazardous reagents, and the depth of modification can usually be confined to the surface only by regulating the exposure time. Further, these techniques usually require expensive equipment and sophisticated process controls. These techniques also involve safety hazards including electric shock, UV exposure, and laser exposure. Further, these surface modification techniques result in non-homogenous surfaces that have multiple surface functional groups, which reduces their selectivity for subsequent derivatization. These techniques are increasingly unacceptable from an environmental and safety perspective. These techniques also have difficulties in modifying the surface uniformly and reproducibly.
Some emerging materials applications require molecular level control of the spatial distribution of chemical functionalities comprising the surface. Several patterning techniques have been demonstrated, including nanografting, microstamping, and photolithography. However, several limitations are associated with each of these methods. Nanografting requires the use of atomic force and scanning tunneling microscopes and is a slow technique requiring physical contact with each spatial location in the pattern. Microstamping requires fabrication of both a positive and negative mold and involves subsequent alignment of the stamps and a mechanical step to transfer physisorbed monolayers to the surface of the substrate. Photolithography involves the use of a patterned photo-mask containing opaque and translucent regions to choreograph regiospecific photochemical changes within a photoresist. The resolution of the patterned area is limited only by the wavelength of the light, but existing techniques suffer from the usual requirement of a development step to create the pattern.
Accordingly, there is a great need for inexpensive, convenient, and accessible methods of selectively modifying surfaces. There is also a need for molecular-based methods and processes that can be used to design and control the chemical and physical nature of surfaces and interfaces. It is desirable to modify functional groups in a homogenous way. It is also desirable to control the spatial distributions of surface functional groups.